The greenhouse industry is experiencing an era of rapidly advancing technologies for artificial illumination. LED based luminaires have entered commercial use as grow lights relatively recently. HPS and conventional arc light sources are now moving aside and more efficient LED luminaires are emerging into markets including advanced functionalities, e.g., integrated pest management (Vänninen et al., 2012).
However, the potential modes of LEDs for illuminating plants are still rarely fully optimized. Currently used LED based luminaires still suffer low efficiency and provide emission spectra not well overlapping with the absorption spectra of photobiological processes such as photosynthesis. Over-exposing of plants with high intensity sources and lack of advanced control modes such as pulsed illumination are still topics not fully researched or solved in practice. An LED spectrum can be matched with photobiological requirements to enhance plants' growth and to increase the total organic output, i.e., the harvested volume of greenhouse products, e.g., tomatoes or lettuce.
Photobiological requirements are mainly defined by the absorption spectrum of the photosynthesis and other photobiological processes in question.
There is also a need to meet the timing requirements of the illumination when operating with a pulsed light. The timing requirement arises from the chlorophyll B excitation and electron transfer delay to the chlorophyll A associated process and the potential to optimize the energy usage for driving the photosynthesis. Other natural parameters that account for the illumination requirements include, e.g., partial pressure of carbon dioxide, irrigation level of soil, temperature and type of canopy. Other requirements that constitute to the required illumination spectrum may arise, e.g., from marketing motives to grow vegetables with certain skin colors or the need to enhance the product's nutrition content or other effective substance.
Different plants and biomass applications require slightly differing type of illumination conditions to reach optimal growth. This induces the greenhouse industry to invest in many types of artificial grow lights. It is the objective of the disclosed invention to provide an integrated LED structure with adjustable emission characteristics to meet the different requirements of various biomass growing applications.
A good example is, e.g., the growth of red and black soybeans. According to CN103947470A and CN103947469, particular light spectrum conditions are preferred for optimum growth of red and black soybeans, with rough blue, red, and yellow spectrum band ratios being 3:1:5 and 4:3:3 respectively, demonstrating the need for adjustable spectrum type light sources to enable one artificial grow light to be used with a variety of different plants. Similarly for example tomato plant and spruce require quite different types of light to grow efficiently. The required spectrum components also vary between different growing cycles of a same plant, e.g., during a vegetation phase blue rich light is preferred, and flowering and fruit grow phases are typically connected with red rich light. Another requirement for adjusting the spectrum of the grow light is the need to grow, e.g., vegetables with varying skin colors of, e.g., bell paprika for marketing purposes or for enhancing certain nutrition components in the paprika fruit.
A grow light with an adjustable spectrum would also allow new functionalities not yet fully exploited in the greenhouse industry. For example, it is known that a pre-harvesting treatment of kale affects strongly on the nutrition content (Carvalho et al., 2014; Lefsrud et al., 2008). Another example is the UV flash-treatment of cultivated mushrooms prior harvesting or post harvesting to enrich their vitamin D content (Beelman et al., 2009).
Another example of potential benefits of a source with an adjustable emission spectrum becomes apparent when considering biomass growth applications such as algae (Nicklish, 1998). The absorption spectrum shifts from around a 680 nm peak towards a lower wavelength peak around 630 nm when the photoperiod becomes shorter. Similar shifts in absorbance are documented in the literature (Eytan, 1974). The ratio of chlorophyll A and chlorophyll B concentration has been shown to change in time when plant is subjected to continuous illumination as, e.g., in case of red kidney bean plants (Argyroudi-Akoyunoglou, 1970). Such change presupposes an alteration in the emission spectrum to maintain optimum growth conditions.
It is clear that a grow light should allow flexible modification of spectrum characteristics to enable its use for growing different types of plants and even modifying spectrum characteristics during the different growth phases. These requirements combined with the idea of growing biomass with a pulsed light source are now tackled with the disclosed invention.
Two main approaches exist to build a LED source for luminaires used as grow lights.
In the first approach, the emission spectrum can be generated by combining optical output of different color discrete LEDs. This type of hybridized LED structure is often called an RGB LED. In this approach the LEDs are discrete LED components and, e.g., blue-red emissions have clearly distinct spatial source points. The light is produced within a compound semiconductor pn-junction while the emission spectrum from a single pn-junction is relatively narrow, typically only 10 to 40 nm. Due to the narrow emission spectrum, several semiconductor chips are used in combination to provide the required wider spectrum to fully cover the red and blue wavelength bands of the visible spectrum required by, e.g., photosynthesis. The required semiconductor chips can be packaged discretely or mounted inside a same package, however optically forming still a large source point.
In the second approach the emission spectrum is generated within a single LED package. In this case one or several LED semiconductor chips excite a wavelength conversion material or typically a phosphor material layer to generate a continuous emission spectrum matching closely with the photobiological requirements. For example, 425 nm LEDs chip excite an appropriately selected phosphor material layer and can provide a typical double peak spectrum offering a relatively good match with the above explained requirements with the primary photobiological process of photosynthesis. One such phosphor material is based on nitridoaluminates and provides a narrow band emission spectrum with a Full Width Half Maximum (FWHM) of 50 nm or less and matching well the absorption spectrum of chlorophyll molecules.
In short, commercial light sources, being LED, fluorescent, or HPS, all still commonly apply continuous light with fixed optical spectrum. It is known that it would be beneficial to apply pulsed light to firstly save energy and secondly to apply a light source that would enable spectrum adjustment to meet changing spectral requirements during plant growth cycles, or phases of photosynthesis, or to allow the use of the same luminaire supporting varying light requirements. A pulsed light arrangement has been shown to also benefit algae growth (Sforza et al., 2012).
PPF (photosynthetic photon flux) should be kept at levels similar or equal to a sun light level that is roughly 2000 μmols/m2/s to avoid excess light and stressing plants. This applies for continuous light. With pulsed light the situation changes as the dark cycle can be adjusted so that the photobiological process has time to ‘use’ the light energy absorbed during the light cycle. Thus, the maximum light intensity can be increased substantially from the nominal sun light level of 2000 μmols/m2/s to 10000 μmols/m2/s to allow even faster growth. However, such arrangements presume considering the excess heat from the light source, other growth limiting parameters such as the level of carbon dioxide, and also how to avoid self-shadowing from the canopy to best utilize high intensity source.
Artificial grow lights have been under research and development for decades (Olle et al., 2013; Klueter et al. 1980; Yeh et al. 2009). Also, pulsed light sources have been introduced earlier (JP S6420034A). This source was based on discharge lamps and was able to produce pulse lengths between 1 to 50 ms. This early innovation was impaired by the fact that the discharge lamps did not well meet the required spectrum characteristics because a large part of the light energy is emitted at wavelengths not needed by photobiological processes. Furthermore, the pulse lengths were not short enough to fully exploit the benefits of pulsed light.
A study carried out by Tennessen with co-workers (Tennessen et al., 1995) shows the benefits of pulsed light. In this study, a pulse period of 100 μs and dark periods of a few ms were used. The experimental light source was assembled from discrete LED components emitting at narrow fixed wavelength bands of 658/668 nm only.
A first pulsed grow light based on LEDs appears in U.S. Pat. No. 5,012,609 (Ignatius). This approach was based on discrete emitters for each required wavelength band, i.e., 400-500, 620-680, and 700-760 nm. The driving circuit was able to produce pulses in duration of 100 μs, i.e., at optimum length. However, the driving circuit was based on a current-limiting-resistor and is considered to have modest energy efficiency when compared to modern solutions such as the one of the disclosed invention. The main drawback of the approach was that it did not provide means to adjust the spectrum for different growth cycles. The spectrum was fixed because the discrete visible range wavelength emitters were all required to be in the same serial-parallel circuit.
U.S. Pat. No. 5,278,432 (Ignatius) presents some innovations regarding the packaging and mounting of discrete LEDs on a heat sinking substrate. However, the driver circuit is still in the form of current-limiting-resistor and the spectrum is fixed with all emitters coupled in series-parallel fashion, excluding the possibility to somehow control the intensity at certain wavelength bands or to adjust the emission spectrum.
WO 02/067660 discloses a system level arrangement of red and white light LEDs to optimize the emitted spectrum to speed plant growth. In the disclosed structure, the spectrum is fixed after the discrete LEDs have been mounted on the carrier substrate. It is clear from this and later publications discussed below that pulsed light is a preferred mode of operation to reduce the total growth time.
The AC driver arrangement disclosed in US 2010/0244724 (Philips) provides a means to reduce total cost of the system by applying same driver circuit for two discrete light sources, emitting in opposite phases of the sinusoidal AC current. An obvious issue is the spatial separation of the two LED strings to avoid over exposing the plants and to gain the benefits of the pulsed lighting.
U.S. Pat. No. 8,302,346 (UoG) discloses a growth enhancing system with a feedback based arrangement applying a pulsed light source based again on discrete LED chips each emitting a fixed spectrum.
CN 201797809 discloses a light source arrangement that applies discrete LED emitters to form the required total spectrum including UV, UVB, blue and near IR.
CN103947470 and CN103947469 disclose light spectrum conditions preferred for optimum growth of black and red soybeans, with rough blue, red, and yellow spectrum band ratios being 3:1:5 and 4:3:3 respectively, demonstrating the need for adjustable spectrum type light source to enable wider use for growth of different plants.
CA 2,856,725 discloses a hybridized light source arrangement that would allow spectrum tunability and pulsed operation mode to prevent photosynthesis saturation. However, the presented light source structure has a system level approach based on discrete LED components mounted on printed circuit board with different emission wavelengths, and with a fixed ratio of LED emitters at individual wavelength ranges to create required spectrum. The expensive feedback system, based on absorption and/or fluorescence sensing, gives coarse feedback to allow tuning of intensity, and of light on and off periods, i.e., the light patterns. However, as the absorption of other than chlorophyll molecules such as carotenin molecules play important role in a plant's heat sinking capability, an effectively large part of light energy is wasted when absorbance is used as a feedback.
WO 2014/188303 discloses a means for enhancing plant growth by adjusting the ratio of blue and red lights alone. US 2014/152194 (Beyer) discloses another system to be able to provide necessary spectrum bands for enhancing growth.
US 2013/318869 has fixed intensity ratios of characteristic peaks at wavelength bands of 400-500 nm (blue), 500-600 nm (green), 600-800 nm (red), and with the 500-600 nm band having a lower intensity compared to other two. However, this arrangement does not allow adjusting the ratio between the intensities of the blue and red wavelength bands.
US 2014/034991 and US 2006/261742 both disclose similar LED arrangements that enable the tuning of the color coordinates and thus the chromaticity of the light emitted from the LED arrangement. However, these arrangements do not address the requirements of biomass growing applications or, e.g., pulsed light operation. The emission spectrum does not meet photobiological requirements. The operation is defined to be continuous, while not meeting the requirement of having alternating emission spectrums of pulsed type.
WO 2013/141824 discloses a similar LED arrangement that enables tuning of the spectrum for matching chlorophyll A and B absorbance. However, the arrangement is not addressing other requirements of biomass growing applications, such as pulsed light operation. The operation is defined to be continuous, thus failing to benefit from an alternating emission spectrum.